1. Field of Invention
The present invention relates to an exhaust gas purifying catalyst. More specifically, the present invention relates to an exhaust gas purifying catalyst in which two different oxygen storage materials are used in a specific combination to achieve a high level of NOX purification performance. The term “oxygen storage capacity” is hereinafter occasionally abbreviated to “OSC”. The term “high level of NOX purification performance” used herein means that a level of NOX purification performance is equal to or higher than those of exhaust gas purifying catalysts according to related arts.
2. Description of Related Art
Exhaust gas that is discharged from an internal combustion engine of an automobile or the like contains HC (hydrocarbons), CO (carbon monoxide) and NOX (nitrogen oxides), and these substances are emitted into the atmosphere after being purified by an exhaust gas purifying catalyst. As a typical example of the exhaust gas purifying catalyst that is used for this purpose, a three-way catalyst that is composed of a porous oxide carrier such as alumina (Al2O3), silica (SiO2), zirconia (ZrO2) or titania (TiO2), and a noble metal, such as platinum (Pt), rhodium (Rh) or palladium (Pd), that is supported on the carrier is known.
The three-way catalyst, which can purify HC and CO in the exhaust gas by oxidation and also purify NOX by reduction, produces the highest effect in a stoichiometric exhaust gas atmosphere from combustion at an air fuel ratio that is close to the theoretical air-fuel ratio. Improvement of the fuel efficiency leads to an increase in the opportunities for an exhaust gas purifying catalyst to be exposed to sudden fluctuations of atmosphere due to A/F fluctuations at a high temperature that are caused by, for example, an increase of the number of times of fuel cut (FC) at a high temperature. Such sudden fluctuations of atmosphere can significantly accelerate deterioration of the catalyst.
The A/F of the gas into an exhaust gas purifying catalyst can fluctuate significantly depending on the driving conditions such as acceleration and deceleration. In such a case, an oxygen sensor that is located on the side of the exhaust gas purifying catalyst performs a control function to adjust the inside of the catalyst to a stoichiometric state. In this case, the catalyst is required to have a high OSC rate to absorb the A/F fluctuations during transitions. On the other hand, because there are running ranges in which the vehicle speed is not changed so drastically when the vehicle is driven in a drive mode, the catalyst is also required to exhibits its OSC capacity for a long period of time in a situation where the A/F of the gas into the catalyst does not fluctuate so drastically.
In addition, reduction of sulfur components that are contained in fuels, such as gasoline and kerosene, is in progress, but it is impossible to remove sulfur components completely and it is unavoidable for a small amount of sulfur components to remain in the fuels. The sulfur components in the fuels deteriorate the performance of the exhaust gas purifying catalyst. Thus, an exhaust gas purifying catalyst that can maintain its catalytic performance under various conditions is desired. In addition, because the noble metals that are used in exhaust gas purifying catalysts are expensive and limited as natural resources, it is necessary to use less noble metals. Thus, various techniques are used to improve the activity of exhaust gas purifying catalysts.
For example, Japanese Patent Application Publication No. 2008-62130 (JP-A-2008-62130) discloses an exhaust gas purifying catalyst. This catalyst contains an oxygen occluding material that is obtained by mixing first cerium-based oxygen occluding material particles (A) which have a first number average particle size and second cerium-based oxygen occluding material particles (B) which have the same composition as the particles (A) and a second number average particle size which is greater than the first number average particle size. A deterioration time of an oxygen occlusion/release capacity is adjusted to a predetermined length. As a specific example of the catalyst, an exhaust gas purifying catalyst that contains an oxygen occluding material mixture that is obtained by mixing two different types of cerium-based oxygen occluding material particles on which Rh is supported and a Pd-supporting alumina powder is shown.
Japanese Patent Application Publication No. 2009-19537 (JP-A-2009-19537) also discloses an exhaust gas purifying catalyst. This catalyst has an upstream catalyst part that contains a Pd/alumina which is composed of Pd-supporting alumina particles and an Rh/OSC which is composed of Rh-supporting oxygen occluding material particles, and a downstream catalyst part that contains a Pt/alumina which is composed of Pt-supporting alumina particles and an Rh/OSC that is composed of Rh-supporting oxygen occluding material particles. With this arrangement, the mass ratio ZrO2/CeO2 of the oxygen occluding material particles in the upstream catalyst part is greater than the mass ratio ZrO2/CeO2 of the oxygen occluding material particles in the downstream catalyst part. In this catalyst, the oxygen occluding material particles in the upstream catalyst part are highly responsive in occluding and releasing oxygen and have a high oxygen occlusion/release rate, and the oxygen occluding material particles in the downstream catalyst part occludes and releases a large amount of oxygen.
Japanese Patent Application Publication No. 2009-84061 (JP-A-2009-84061) discloses a pyrochlore phase type ceria-zirconia composite oxide in which 50% or more of the pyrochlore phase type regularly-arrayed phase that exists before heating remains in the ceria-zirconia composite oxide even after heating at a temperature of 1000° C. or higher in the atmosphere. In addition, Japanese Patent Application Publication No. 2007-247968 (JP-A-2007-247968) discloses a catalyst material that is composed of particles of two different types of metal oxides including at least one of CeO2, ZrO2, Al2O3, TiO2, SiO2, MgO, Y2O3 and LaO3 on which a noble metal is supported.
However, any of the above-mentioned exhaust gas purifying catalysts does not have satisfactory NOX purification performance after endurance and therefore an exhaust gas purifying catalyst that has higher NOX purification performance is desired.